The present invention relates to a method and an apparatus for manufacturing superfluidity helium, and more particularly to a method and an apparatus for transforming liquid helium (".sup.4 He") to superfluidity helium (He II) by indirectly cooling the liquid helium with cryogenic .sup.3 He gas.
Liquid helium has the lowest boiling point of all elements, and is transformed superfluidity helium (He II), a state having a quite different nature and different properties compared with liquid helium, by cooling the liquid helium to less than 2.18K (the so-called Z point). Superfluidity helium has, among other unique properties, the property of superfluidity, in which the liquid exhibits completely frictionless flow and can pass through extremely small holes with the greatest of ease. In addition, it is well known that superfluidity helium stabilizes the phenomenon of superconductivity (zero electrical resistance at cryogenic temperatures) due to its very high heat conductivity. Superfluidity helium is therefore well suited to be used for cooling the superconducting magnets used in large particle accelerators or test installations for nuclear fusion.
Heretofore, superfluidity helium (He II) has been manufactured by (1) liquefying helium gas to produce liquid helium at standard pressure and a temperature of 4.2K using a conventional Claude cycle refrigerator, (2) adiabatically expanding the liquid helium to a pressure of less than about 0.015 atm, and (3) cooling it using the Joule-Thomson effect to partially transform the helium to He II. However, this method requires the use of several large vacuum pumps (compression ratio: about 67) for compressing cryogenic gas of less than 0.015 atm to standard pressure (1 atm).
For improving the above prior art method, the "TORE SUPRA" in France has used the refrigerating apparatus shown in FIG. 7 (Prior Art). In this apparatus, the helium gas is compressed by a compressor 1 from 1 atm to about 15 atm, and cooled by heat exchangers 2, 3 and 4, respectively, using cryogenic helium of 77K, 13K and 4.2K formed by another Claude cycle refrigerator (not shown). The helium gas is then expanded to about 0.012 atm and cooled to 1.7K using a JT valve 5. Part of the cooled helium gas is transformed to superfluidity helium. The remaining non-liquified cryogenic helium gas is compressed to about 0.05 atm by a cryogenic temperature centrifugal vacuum pump 6, further compressed to 1 atm by a large normal temperature reciprocal vacuum pump 7, and finally returned to compressor 1.
However, in the method and the apparatus mentioned above, the pressure of the helium gas must be reduced to a low pressure vacuum state of less than about 0.015 atm since this method and apparatus cool the helium gas to less than 2.18K (the .lambda. point), preferably less than 1.8K by using the helium's own adiabatic expansion. Accordingly, the use of a large vacuum pump is unavoidable.
In addition, since the compression ratio of the reciprocal vacuum pump 7 is high (about 20) and thus is liable to cause leakage of the helium gas, the pump 7 is sealed by using a lubricant which would cause contamination of the helium gas. Accordingly, it is necessary to provide a large purifier in order to maintain the helium gas at a high purity.
Furthermore, it is necessary to cool the superfluidity helium at standard or near standard atmospheric pressure. This helium is called "pressurized superfluidity helium" since it is pressurized higher than its boiling point vapor pressure. The standard pressure helium is cooled by a heat exchanger in order to obtain pressurized superfluidity helium, since the pressure of superfluidity helium obtained by the method and the apparatus of the prior art is substantially less than standard pressure (e.g., less than 0.015 atm). At a low pressure near the boiling point of the superfluidity helium, the superfluidity helium is immediately vaporized when the heat load is high and therefore sufficient cooling with the superfluidity helium cannot be achieved. Accordingly, the method of FIG. 7 (Prior Art) uses a method of (1) indirectly cooling the superfluidity helium (i.e., pressurized superfluidity helium) sealed at or near standard pressure, and (2) indirectly cooling a superconducting magnet 8 arranged at a remote place as shown in FIG. 7 using the very high superconducting characteristics of the pressurized superfluidity helium. The indirect cooling is conducted without circulating the pressurized superfluidity helium. However, this method has the problems (1) that the heat exchangers must be large due to the inferior heat conductivity of the helium gas in a low pressure vacuum state, and (2) that the cooling capability is limited due to the limited heat conductivity of the pressurized superfluidity helium.
It is an object of the present invention to provide a method and an apparatus for manufacturing superfluidity helium which can solve the above problems of the prior art.
Specifically, it is an object of the present invention to provide a method and an apparatus for manufacturing superfluidity helium which does not require a large vacuum pump and therefore can avoid the necessity of using a large purifier to remove contaminants from the helium gas caused by the lubricant of the vacuum pump.
It is another object to provide a method and an apparatus for manufacturing pressurized superfluidity helium which can cool a superconducting magnet by directly contacting the magnet with superfluidity helium or circulating superfluidity helium around the magnet.